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22 Nickel Based Catalysts Disadvantages: The main problem that renders these catalysts difficult to be used widely is that they are easily deactivated if they are used repeatedly in high temperature. Used nickel catalysts are toxic, so their disposal is a potential environmental problem (Dayton,2002). One of the most important reasons for their deactivation is fouling due to physical blockage by coke. They can also be easily poisoned by sulfur compound for operating temperatures below 900 °C. Sintering at high temperatures is also a common, which reduces the catalyst activity (Rönkkönen,2011b). Regarding mechanical deactivation, attrition causes the loss of the active surface area through crushing. Besides, they are expensive, so long lifetime is required (Abu El-Rub,2004), (Pfeifer,2008). Therefore their use is not recommended for in-situ use because Ni-based catalysts become quickly deactivated, as coke built up is more rapid inside the gasifier. Types of Ni-based Catalysts (Corella,2004): a) Commercial particulate-shaped catalysts (rings, spheres, pellets, extrudates) that require particulate-free fuel gas. b) Un-commercialized monoliths (honeycomb structure) that have been not studied thoroughly. Further investigation would be interesting as it has been reported that can operate with fuel gas containing particulates, avoiding the use of filters, though they are noted very active, have a high cost and their technology is difficult. They also have to operate in adiabatic form (Rönkkönen,2011b). The most practical catalyst structure should probable be a honeycomb monolith catalyst. In this type the pore diffusion resistances are much smaller than in the pellet catalysts. Additional Characteristics of Ni-catalysts are proved through tests, such as the fact that all the components of the gas utilize the same active nickel sites on the catalyst surface. Also it has been reported that the pressure drop is lower for shapes with internal holes (Hepola,1997a). According to some tests that were realized (Aznar,1993) the particle size of the catalyst gas also affects slightly its effectiveness. As the diameter of the particle decreased, less tar yield and more gas yield were obtained in the outlet gas due to a slight diffusion control for dp> 1.6 mm. In addition, with the increase of particle size more carbon is being deposited (Rönkkönen,2011b).3.1 Deactivation Causes of Ni-based Catalysts The activity of a catalyst is very crucial in order to be widely applied in the industry. Three are the most common reason which causes low activity. 3.1.1 Sulfur poisoning Sulfur is adsorbed on the surface and the steam reforming reactions can no longer occur so the catalyst is deactivated, as seen below in Figure 11. Sulfur may cause significant deactivation even at very low concentrations, due to the formation of strong metal-S bonds. Figure 11: Sulfur poisoning of Ni-based catalysts http://resources.schoolscience.co.uk/JohnsonMatthey/page21.htm (28)
Nickel Based Catalysts 23 The content of sulfur adsorbed on the catalyst surface depends on temperature, pressure, H2S concentration in the gas and the catalyst type (Hepola,1997a). Regarding the catalyst type, sulfur adsorption capacity depends on the nickel surface area, the shape of the catalyst pellet. During the poisoning the bed temperature decreases considerably because steam decomposition is an endothermic reaction and then after poisoning it is increased again. Therefore, poisoning can be indicated by the determination of the increase of the temperature (10-40 °C) at the top of the catalyst bed. In addition, higher hydrocarbons, although they have larger molecular sites than methane, their conversion is less affected by sulfur poisoning than that of methane. The methane content is larger than the tar content in overall so the need of active nickel sites is higher. According to some tests homogenous Ni-Catalysts are affected less by sulfur. Bulk nickel sulfide may also form in special cases. When bulk nickel is formed, toluene decomposition decreases because the H2 component from the H2S species interacts with the unsaturated hydrocarbons to give hydrogenated species. Also the concentration of sulfur seems almost the same at all parts of the bed at this state (Hepola,1997a). Sulfur formation is evident inside the pores of the catalyst as seen in Figure 12 (Lassi,2003): Figure 12: Sulpfur formation (Lassi,2003) Effect of the different processes parameters on sulfur poisoning: Temperature: Based on thermodynamics, sulfur formation is an exothermic reaction. Thus, according to Le Chatelier’s principle at lower temperatures the reaction towards sulfur poisoning of the catalyst surface is favored. So, the higher the temperature the lower the sulfur poisoning effect. It is recommended for catalysts to operate at >900°C. At these conditions no decline in decomposition activity is reported (Hepola,1997b).At temperatures >900°C the amount of sulfur adsorbed forms an irreversible monolayer on the catalysts surface. For temperatures
Page 1: Betreuer der Arbeit: Dipl. Ing. Mat
Page 5: Acknowledgements This work was acco
Page 8 and 9: II Content Content Acknowledgements
Page 10 and 11: IV List of Figures List of Figures
Page 12 and 13: VI List of Abbreviations List of Ab
Page 14 and 15: VIII Notation Notation ΔH [Kj/mol]
Page 17 and 18: Introduction 1 1 Introduction The d
Page 19 and 20: Introduction 3 1.2 Tasks The presen
Page 21 and 22: Biomass Gasification 5 2 Biomass Ga
Page 23 and 24: Biomass Gasification 7 Spouted Bed
Page 25 and 26: Biomass Gasification 9 2.2 Gas Clea
Page 27 and 28: Biomass Gasification 11 types of ca
Page 29 and 30: Biomass Gasification 13 Mechanical
Page 31 and 32: Biomass Gasification 15 Partial oxi
Page 33 and 34: Biomass Gasification 17 The followi
Page 35 and 36: Biomass Gasification 19 Groups of C
Page 37: Nickel Based Catalysts 21 even show
Page 41 and 42: Nickel Based Catalysts 25 ing press
Page 43 and 44: Nickel Based Catalysts 27 3.4 Proce
Page 45 and 46: Iron Based Catalysts 29 4 Iron Base
Page 47 and 48: Iron Based Catalysts 31 Measurement
Page 49 and 50: Iron Based Catalysts 33 The percent
Page 51 and 52: Iron Based Catalysts 35 4.6 Catalys
Page 53 and 54: Iron Based Catalysts 37 Reduction i
Page 55 and 56: Iron Based Catalysts 39 and MgO (Di
Page 57 and 58: Precious Metal Catalysts 41 5 Preci
Page 59 and 60: Precious Metal Catalysts 43 Bimetal
Page 61 and 62: Experiments 45 Table 3: Biomass fee
Page 63 and 64: Experiments 47 6.3.1Experimental Pr
Page 65 and 66: Experiments 49 6.3.2 Tar Sampling M
Page 67 and 68: Results and Discussion 51 7 Results
Page 69 and 70: Results and Discussion 53 7.2 Sampl
Page 71 and 72: Results and Discussion 55 Wet Gas C
Page 73 and 74: Results and Discussion 57 er, the v
Page 75 and 76: Results and Discussion 59 7.3.1. Ef
Page 77 and 78: Results and Discussion 61 The conve
Page 79 and 80: Results and Discussion 63 7.3.3 App
Page 81 and 82: Results and Discussion 65 7.4 Blank
Page 83 and 84: Summary/Outlook 67 The aim is to pr
Page 85 and 86: References 69 (Aznar,1993) Aznar, M
Page 87 and 88: References 71 (Hepola,1997b) Hepola
Page 89 and 90:
References 73 (Sarvaramini,2012) Sa
Page 91 and 92:
75 Appendix Appendix A: Iron based
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Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

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